Fuel gauge calibration

[Breakin Away]

...If zero, 50% (in this case inches, not gals,) and100% isn't within the acceptable variance, I throw out the calibration and investigate the cause because it could have a shift, or a clip, or be out of scale, or be damaged that it needs to be serviced/replaced.

OK, after thinking about it and re-reading several times, I see what you are saying. I think our difference in interpretation is that I have much lower expectations of a fuel gauge. Every car I've had over the last 20 years had a fuel gauge that dropped like a rock once below 1/2 a tank. So a fuel gauge that says 1/2 but only has 9-10 gallons (out of 25 gallon capacity) is no big deal to me. Also, this might be a direct result of the bottom aft corner of the tank being "truncated" to conform with the hull shape. All of that is why I built a calibration curve, because I expect the fuel gauge to be inaccurate and want a graph to correct the discrepancy.

Maybe if I pulled the tank out and did surgery I'd find some issue. But as long as it's consistent year-to-year, I'll live with it, just like I've done for my cars, since as you can see by the picture I linked above, that sucker is not coming out without a LOT of potentially destructive work.

[KWKloeber]

BA

That's why I was careful to keep my exanple related to level (0 - 1/2 - full) and not gals. 

Mea culpa for not reading back about the non-standard tank and the (it seems?) inaccuracies/variances in how the data was collected.  For instance if you aren't sure of the total gals of YOUR odd tank (assuming that full is 25 gals (including the fill hose)) then I agree there are many contributors that could produce only a very general calibration.  That would just drive me batty because I wouldn't be able to tie down which of the variables (or all?) is throwing off what I'd expect to find on a data plot.  Jon's pics seem surprisingly accurate given the C34 odd tank shape.

It wouldn''t require pulling the tank, but it would be an effort to drain and carefully determine the precise gals at each eighth gauge reading.

Additionally, maybe the sender isn't even appropriate for the tank you have (I mean the height of the tank vs travel of the float?)  Do you think that might be the cause of the clipping?
 
One never knows what was inherited as far as the tank/sender??  So what you have may be the best you can reasonably get.                 

[Ron Hill]

Guys :To solve the problem - is to have the fuel gage manufacturer have a logarithmic scale on the gage to match the asymmetrical fuel tank!!   

A thought   

[Stu Jackson]

Or to just use fuel consumption.  I haven't had a working fuel gauge in over 20 years.  Golly, somehow I've survived...

[Jim Hardesty]

Or to just use fuel consumption.  I haven't had a working fuel gauge in over 20 years.  Golly, somehow I've survived..

I agree.  My fuel gage works, but I trust the hour meter more.  To keep from going into the log book all the time, I cut down a small white-board and mounted at the nav station.   My hour meter reading from last fill up is posted there along with last pump out date and other things I want to keep in mind.  Real handy and useful for me.
Jim

[Jon W]

When underway the boat is pitching and yawing. Without a stillwell, any reed switch/lever arm sender is going along with the motion. I agree the most accurate measure is fuel consumption/hr. Which to be honest is a guess as well since it depends on rpm, and how hard the engine is working to keep that rpm. I treat the fuel gauge as a visual reference only. I found the fuel gauge photos with gallon reference to be a very handy reminder.

[mark_53]

When underway the boat is pitching and yawing. Without a stillwell, any reed switch/lever arm sender is going along with the motion. I agree the most accurate measure is fuel consumption/hr. Which to be honest is a guess as well since it depends on rpm, and how hard the engine is working to keep that rpm. I treat the fuel gauge as a visual reference only. I found the fuel gauge photos with gallon reference to be a very handy reminder.

Import to keep in mind what's being measured.  The fuel gauge is a measure of fuel LEVEL not capacity.  It's the level of the fuel in the tank as indicated by the float's position.

[Jon W]

That's the point I'm making, there isn't LEVEL on a boat underway.

[Breakin Away]

Guys :To solve the problem - is to have the fuel gage manufacturer have a logarithmic scale on the gage to match the asymmetrical fuel tank!!   

A thought   

Only one problem: A logarithmic scale NEVER GETS TO ZERO    

[Breakin Away]

...Mea culpa for not reading back about the non-standard tank...

The plastic tank was actually the standard tank for several years up to 2004... and I'm actually very happy with it. It does not corrode, and I've had no problems with odors due to fuel permeation - though I'll continue to empty it every winter to reduce the chance of eventual permeation.

It wouldn''t require pulling the tank, but it would be an effort to drain and carefully determine the precise gals at each eighth gauge reading...

Now that I've burned off all my old fuel and turned it over a few times this year by running on empty, I plan to put in 25 gallons (a few gallons at a time) next year to get a more accurate calibration.

I agree with the others that calculating fuel consumption is the most reliable method. I see 0.55-0.57 gph under my normal conditions. The gauge is mostly just a general guide, but if I have data I will continue to plot it.

[KWKloeber]

there isn't LEVEL on a boat underway.

Have we actually seen wildly fluctuating fuel gauges while underway rockin and rollin?  Theoretically there could be but I haven't seen this.  At last not substantially.  My gauge? or sender? (or both?) have sufficient dampening to average out high fluctuations (no baffles in tank.)

We all ALWAYS use the 1/3 fuel rule anyway, right?  So checking level underway isn't even "necessary."  HA!

-k

[scgunner]

Ken,

In regards to needle bounce, because I had been racing I had about 1/4 of a tank. On the way home in about six foot swells the needle bounced so wildly, from empty to full, that I couldn't tell how much fuel was left. I had planned to motor home since I was by myself and towing two dinghies, but decided to sail since I couldn't be sure how much fuel I actually had, that was not a fun sail. The moral of the story, check the fuel gauge in flat water before you depart.